Fracture Analysis on the Popcorning of a Plastic Ball Grid Array Packageduring Lead-free Solder Reflow Process

摘要

Moisture-induced cracking during the solder reflow process is known as 'popcorning', which is a critical reliability issue toplastic packages.In plastic ball grid array (PBGA) packages, the cracking usually occurs at the die attach area.This kind ofphenomenon is mainly resulted from a combination of thermal expansion mismatch between the constituent materials of thepackage and the steam pressure from vaporized moisture absorbed by the package.Therefore, the crack propagation ofpopcorning is a mixed-mode fracture problem in nature.In the present study, a finite element simulation is performed to modelthe popcorning mechanism of a PBGA package during the lead-free solder reflow process.The shear stress is used as afailure criterion for the crack initiation.Fracture mechanics is employed to characterize the crack tip properties after the crackinitiation.Since the crack surfaces are not traction-free, the conventional J-integral formula may not be valid.In order toevaluate the fracture parameters at the crack tip, another approach is adopted.The crack tip opening displacement (CTOD)method is introduced in the present analysis to determine the stress intensity factor (SIF), the phase angle, and the strainenergy release rate at the crack tip corresponding to various temperatures and crack lengths.The relationship between SIFsand CTODs is expressed in a complex variable form.Subsequently, the strain energy release rate can be obtained.It is foundthat Mode II fracture is always dominant during the crack propagation process.Therefore, it is concluded that thermalmismatch is the major driving mechanism for the development of popcorning crack.